Financial Instruments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) position	Dec. 31, 2018 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 93,805	$ 107,461
Other than temporary impairment	$ 0
Number of investments, unrealized loss position | position	12
Amortized Cost
Less than one year	$ 57,974
Greater than one year but less than five years	36,097
Total	94,071
Fair Value
Less than one year	57,910
Greater than one year but less than five years	35,895
Total	93,805
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	94,659	134,552
Unrealized Gain	117	36
Unrealized Loss	(383)	(588)
Estimated Fair Value	94,393	134,000
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	588	26,539
Estimated Fair Value	588	26,539
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	57,857	76,061
Unrealized Gain	62	29
Unrealized Loss	(287)	(405)
Estimated Fair Value	57,632	75,685
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	13,010	13,017
Unrealized Loss	(72)	(126)
Estimated Fair Value	12,938	12,891
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	23,204	18,935
Unrealized Gain	55	7
Unrealized Loss	(24)	(57)
Estimated Fair Value	23,235	18,885
Cash equivalents | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	588	26,539
Short-term investments | Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 93,805	$ 107,461